1. Technical Field
The present disclosure relates to channel estimation in communication systems.
2. Background Information
In a wireless communication system, a transmitter typically encodes, interleaves, and modulates (i.e., symbol maps) traffic data to obtain data symbols. For a coherent system, the transmitter multiplexes pilot symbols with the data symbols, processes the multiplexed pilot and data symbols to generate a modulated signal, and transmits the signal via a wireless channel. The channel distorts the transmitted signal with a channel response and further degrades the signal with noise and interference. The transmitted signal may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors. Different frequency sub-bands used for communication may experience different channel conditions and may have different signal-to-noise ratios (SNRs). An accurate estimate of the response of the channel between transmitter and receiver is therefore normally needed in order to communicate data effectively.
The receiver receives the pilot modulation symbols and processes the received pilot modulation symbols to obtain channel response estimates. Because the pilot modulation symbols have values that are known to the receiver, the receiver can estimate channel response based on a function of the pilot symbol values as received and the pilot symbol values that the receiver knows were transmitted. Once the channel estimate is made, the receiver uses the channel estimate to determine, from the received data modulation symbols, what the originally transmitted data modulation symbols were. The receiver then performs symbol demapping, de-interleaving, and decoding on the recovered data modulation symbols in accordance with the coding and modulation schemes used for the traffic data.
There are several different methods of performing channel estimation. In one method, only a small tile of the overall frequency-to-time frame of modulation symbol values is to be demodulated. This tile involves only a relatively small number of pilot modulation symbol values. These pilot values may be referred to as “narrowband” or “dedicated” pilot values. The receiver uses interpolation to interpolate channel characteristics between points of known channel characteristics given by the pilot values. The resulting channel estimates are used to determine, from the received data symbol values, what the originally transmitted data symbol values were.
In a second method, more numerous so-called “broadband” or “common” pilot modulation symbol values are distributed throughout the frequency-to-time frame. An Inverse Fast Fourier Transform (IFFT) function is used to convert the received pilots values into the time domain. The strongest pilots are identified in the time domain, and these strongest pilots are then zero padded. The zero-padded time domain result is converted into the frequency domain by a Fast Fourier Transform (FFT) function to generate a larger set of channel estimates, one for each modulation symbol value in the frequency-to-time frame. In some circumstances and applications, the channel estimation operations performed by the receiver may consume an undesirably large amount of processing power, may require an undesirably large amount of dedicated hardware to be included in the receiver, and/or may cause the receiver to consume an undesirably large amount of power.